1. Field of the Invention
This invention relates to glide-height disk-testers for testing the surfaces of recording disks, such as disks used in magnetic recording disk drives.
2. Description of the Related Art
Magnetic recording disk drives use magnetoresistive (MR) read heads for reading the recorded data from the disks. The MR heads are extremely sensitive to small physical asperities that project from the surface of the disk. If the MR head contacts an asperity there is a momentary frictional heating of the MR element. This heating, called a thermal asperity, increases the resistance of the MR element, which causes data errors and loss of information in reading the disk. In addition, asperities can also physically damage or scratch the read or write head.
To assure that the disks are free of asperities that would project high enough to be contacted by the MR head, one of the final steps in the manufacturing of disks is a glide-height test. The disk must have a qualified glide height (QGHT) to be acceptable, meaning that no asperities should be higher that QGHT. In the conventional glide-height disk-tester the disk is placed on a spin stand and rotated at an initial high speed. A glide slider (also called a glide head), similar to the slider that supports the MR head in the disk drive but typically without a read/write head, is maintained above the surface of the rotating disk and moved radially across the disk surface as the disk rotates. The slider contains a contact sensor, such as a piezoelectric element, that generates an electrical signal when the slider contacts an asperity. With the disk rotating at its initial high speed, the slider is initially flying higher than any expected asperity. The rotational speed is then continuously reduced, which reduces the fly height of the slider, until an asperity, or a predetermined number of asperities, are detected. The glide height at the time of asperity detection is determined from the known disk rotational speed and the radial position of the slider at the instant of asperity contact. The relationship between the linear velocity of the disk relative to the slider (determined from disk rotational speed and radial location of the slider), is well known and can be previously calibrated for a particular slider design.
This method of determining glide height from the known linear velocity of the disk relative to the slider is less reliable as flying heights in disk drives become reduced to tens of nanometers or less. For example, commercially available disk drives now have flying heights of 6–8 nm. In addition, the output signal from the contact sensor is affected by the velocity at impact with the asperity, so that if different asperities are contacted at different velocities, it is difficult to estimate the relative sizes of the asperities. Also, because the linear velocity is not the same for each asperity contact, the pitch and roll of the slider is different for each asperity contact, which makes it difficult to compare different signals from the contact sensor.
What is needed is a glide-height disk-tester that does not rely on the relationship between disk linear velocity relative to the slider and fly height of the slider to determine glide height at the instant of asperity contact.